The invention relates to a method and apparatus for treating lime sludge, and a particular configuration of a rotary lime kiln. Lime kilns and associated equipment for supplying lime sludge to the lime kiln, are standard and important pieces of equipment in the production of cellulose pulp. A lime kiln is part of the pulp mill chemical recovery plant, the lime being used for causticizing green liquor to produce white liquor in the production of kraft pulp.
Lime sludge is a sludge of calcium carbonate produced in the causticizing plant of a sulphate pulp mill. The lime sludge (CaCO.sub.3) is regenerated by reburning it to form calcium oxide (CaO). The reburning takes place in an ordinary rotary kiln, into the upper end of which the lime sludge is supplied. The sludge flows slowly downwardly through drying, heating and reaction zones. Conventional rotary kilns desirably enhance the heat transfer from the flue gases and the lime sludge in the drying zone by providing chains which rotate with the kiln and come into contact with the lime sludge during that rotation. In order to obtain proper results the heat treatment of lime sludge typically takes place slowly in the kiln, meaning that the kiln must be long and therefore takes up significant floor space or land area.
Prior to feeding the lime sludge to the kiln, the sludge is usually dried using mechanical filters. Conventionally the lime has been dried to a dry solids content of between about 60-70%, but using most modern technology a dry solids content of between about 80-90% may be obtained. When sludge having a dry solids content of about 80-90% is used, the dust content of the flue gases of the kiln rises significantly. The moving chains in the kiln detach the dry sludge more easily than less dried sludge, thus causing entrainment of the larger volume of the lime in the flue gases. This also can result in a higher exhaust temperature of the flue gases, which can have problems associated therewith, and desired reduction in fuel consumption in the rotary kiln is not achieved.
One prior art method for eliminating the problems associated with chains used in the rotary kiln, and to allow a shorter kiln, is a suspension-type drying system. In such systems lime sludge is introduced into a substantially vertical conduit through which the flue gases from the kiln move upwardly at a relatively high rate of speed. Most of the lime is entrained in the upwardly moving gas, and dried by contact therewith, and the flue gas with entrained particles is fed to a conventional separator (such as a cyclonic separator), the flue gases being discharged and the lime particles--which now have been dried--being fed to the inlet to the lime kiln. A baffle may be provided which allows lime sludge to be dried in the suspension dryer, fed directly to the kiln, or both, depending upon the dry solids content and particle size of the lime sludge. Such a system is shown in U.S. Pat. No. 5,213,496, the disclosure of which is hereby incorporated by reference herein. Such systems--namely combinations of suspension-type dryers and rotary kilns--are replacing rotary kilns per se in the marketplace because the combination of a suspension-type dryer and rotary kiln provides high heat capacity and good heat economy. Capacity and heat economy of such a combination may be further improved by employing two subsequent suspension drying stages, one of which serves as a dryer per se, the other acting as a preheater. The flue gases from a kiln are first taken to a suspension preheater and from there to the dryer. The lime sludge to be dried is supplied from a lime sludge filter to the dryer, then to the preheater, and finally to the kiln to be combusted therein.
A common problem in such systems is that since the dried lime sludge to be burned is introduced into the same end of the rotary kiln from which the flue gases are discharged, a portion of the sludge is entrained with the flue gas. Up to 5-15% of the lime sludge can be recirculated in this way, which reduces the operational capacity of the rotary kiln, as well as having an adverse affect on the heat economy of the system. Adverse entrainment can also occur when lime sludge coming from a lime filter and having a very high dry solids content (80-90%) is fed directly to the kiln, or when lime sludge dust from the electric precipitator of the kiln is returned back to the kiln.
According to the present invention the problems associated with the prior art system have been overcome, resulting in enhanced heat economy and other advantageous operations of the rotary lime kiln. According to the invention the flow of lime sludge into the kiln first end is provided in a separate path than the flow of flue gases out of the kiln, so that contact between the lime sludge entering the kiln and flue gases exiting it is substantially prevented.
According to one aspect of the present invention a method of supplying lime sludge to a rotary kiln having a first end into which lime sludge to be burned is introduced, and from which flue gases from burning in the kiln are discharged, is provided. The method comprises the steps of: (a) Guiding the flow of lime sludge into the kiln first end in a first path. And, (b) guiding the flow of flue gases out of the kiln first end in a second path distinct from the first path so that contact between lime sludge entering the kiln and flue gases exiting the kiln is substantially prevented. Typically a substantially vertical flue gas conduit extends upwardly from the first end of the rotary kiln. There may be the further step, before step (a), of drying the lime sludge, utilizing any suitable or conventional technique.
While a wide variety of different mechanisms for guiding the flow of the lime sludge and the flow of the gases out of the kiln may be provided, such as baffles, valves, shrouds, and the like, preferably steps (a) and (b) are practiced by: (i) providing a generally horizontal tubular duct interiorly of the kiln first end and extending into the substantially vertical flue gas conduit, an annular chamber defined between an outer surface of the tubular duct and an inner surface of an inlet to the first end of the rotary kiln; and (ii) introducing the lime sludge into the annular chamber. Steps (a) and (b) may be further practiced by: (iii) providing spiral feeding blades on the outer surface of the tubular duct, and (iv) rotating the tubular duct with the rotary kiln so that the spiral feeding blades engage the lime sludge and move it toward the kiln. Substep (ii) may be practiced by: feeding wet lime sludge into the vertical flue gas conduit above the rotary kiln so that a significant amount of wet lime sludge is entrained by the flue gas flow, and the wet lime sludge is dried by the flue gas flow; separating the dried lime sludge from the flue gas flow; and transporting the separated dried lime sludge to the annular chamber for introduction into the annular chamber. Substep (ii) may be further practiced by causing some wet lime sludge to drop down into the vertical flue gas conduit adjacent the tubular duct, and mixing the wet lime sludge with the dried lime sludge using the spiral feeding blades.
The method may also comprise the further step of (c) causing any flue gases which enter the annular chamber to exit the annular chamber and flow into the substantially vertical flue gas conduit adjacent a bottom portion of the annular chamber. Step (c) may be practiced by providing a baffle at a top portion of the annular chamber adjacent the vertical flue gas conduit.
Substep (ii) may be practiced by introducing the lime sludge at a vertical position approximately at a vertical midpoint of the tubular duct. Step (b) may be further practiced by providing a portion of the tubular duct extending into the rotary kiln a distance of at least one meter to a remote end, and by tapering the tubular duct between the remote end and the substantially vertical flue gas conduit so as to improve guiding of the flue gas flowing through the tubular duct, and thus ensure that little or no flue gas passes into the annular chamber.
According to another aspect of the present invention apparatus for treating lime sludge is provided comprising the following components: A rotary lime-burning kiln having a first end into which lime sludge is introduced, and from which flue gases from burning lime sludge within the kiln are discharged. First means for guiding the flow of lime sludge into the kiln first end in a first path. And, second means for guiding the flow of flue gases out of the kiln first end in a second path distinct from the first path so that contact between lime sludge entering the kiln and flue gases exiting the kiln is substantially prevented.
The first and second means may comprise a generally horizontal tubular duct interiorly of the kiln first end, and extending into the substantially vertical flue gas conduit, with annular chamber defined between an outer surface of the tubular duct and an inner surface of the first end of the rotary kiln, and a conduit for introducing the lime sludge into the annular chamber. The apparatus may further comprise spiral feeding blades on the outer surface of the tubular duct; and means for rotating the tubular duct with the rotary kiln so that the spiral feeding blades engage lime sludge and move it toward said kiln. The apparatus may further comprise means for feeding wet lime sludge into the vertical flue gas conduit above the rotary kiln so that a significant amount of wet lime sludge is entrained by the flue gas flow, and the wet lime sludge is dried by the flue gas flow; means for separating the dried lime sludge from the flue gas flow; and means for transporting the separated dried lime sludge to the annular chamber for introduction into the annular chamber.
The apparatus may further comprise means--such as baffles, shrouds, screens, diverters, or the like--for causing any flue gases which enter the annular chamber to exit the annular chamber and flow into the substantially vertical flue gas conduit adjacent a bottom portion of the annular chamber. Preferably such causing means comprise a baffle at a top portion of the annular chamber adjacent vertical flue gas conduit. The conduit for introducing lime sludge into the annular chamber may comprise means for introducing the lime sludge at a vertical position approximately at a vertical mid-point of the tubular duct.
According to yet another aspect of the present invention a rotary lime kiln is provided. The lime kiln comprises: A hollow kiln body disposed at a slight incline to the horizontal, and having a first, higher, end, and a second, lower, end, the kiln body rotatable about a central axis having substantially the same slight incline to the horizontal. A tubular duct connected to the kiln body, interiorly thereof, for rotation therewith, the duct having a first duct end extending outwardly from the kiln first end, exteriorly thereof, and a second duct end located within the kiln body at least one meter (e.g. two meters) from the kiln first end. And, spiral feeding blades disposed on an outer surface of the duct at least on part of the first end thereof which extends outwardly from the kiln body first end, exteriorly of the kiln body. The tubular duct may be tapered between the first and second ends thereof, and have a conical portion which increases in diameter from closest the first end of the duct toward the second end of the duct. The kiln may further comprise a plurality of transport blades connected to the outer surface of the duct within the kiln body at least adjacent the conical portion.
It is the primary object of the present invention to provide a lime kiln, and a method and system for supplying lime sludge to a lime kiln, having enhanced heat economy and other advantages compared to conventional processes and equipment. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.